CHANGES IN LIVE PLANT BIOMASS, PRIMARY PRODUCTION AND SPECIES COMPOSITION ALONG A RIVERSIDE TOPOSEQUENCE IN ARCTIC ALASKA, USA

In the arctic landscape, vegetation composition and structure are stro ngly affected by topographic position and associated variation in micr oclimate. Along a single riverside toposequence in northern Alaska, si x distinct plant communities were studied including a riparian shrub c ommunity, a wet sedge tundra, a footslope Equisetum community, a hills lope shrub/lupine community, a hilltop birch-heath community, and a mo ist tussock tundra. Total live plant biomass varied threefold along th e toposequence (450-1400 g m(-2)) while live vascular plant biomass (i ncluding belowground stems and rhizomes but not roots) varied sevenfol d (160-970 g m(-2)). Aboveground vascular plant production varied less than fourfold (80-265 g m(-2)). Although all six communities showed s ome signs of nutrient Limitation, measures of soil nutrient availabili ty were highly variable among communities. Contrary to expectations, t he most productive community along the toposequence was the hillslope shrub/lupine community, where a late-lying snowbank delayed the start of the growing season by 2 wk. The second most productive community wa s the hilltop birch-heath, which was exposed to winter winds and where snow melted earliest; most of the production in this community occurr ed in relatively protected depressions where there were dense accumula tions of plant mass. A conclusion is that soil fertility, soil thaw, a nd protection from wind are more important than length of the snow-fre e season in regulating productivity along the toposequence. Also contr ary to expectations, overall production:live biomass ratios of the six communities varied little despite large differences in growth form co mposition among communities and in biomass turnover among growth forms . High-biomass, highly productive communities had the lowest productio n:live biomass ratios, and thus the lowest biomass turnover. Because p roduction and live biomass were linearly correlated over the entire ra nge sampled, it may be possible to use live biomass and/or leaf area a s a reasonably accurate predictor of productivity at the landscape or regional level in the Arctic.